System and method for maintaining proper termination and error free communication in a network bus

ABSTRACT

A system for maintaining proper termination and error-free communicaton in a network bus ( 12 ) including a power bus includes at least one network device ( 18 ), a network controller ( 16 ) and at least on bus protection element. The network devices ( 18 ) are electrically connected to and adapted to communicate via the network bus ( 12 ). The network controller ( 16 ), in turn, is electrically connected to the network bus ( 12 ) and is adapted to direct communicaitons with the network devices ( 18 ) via the network bus ( 12 ). The network controller ( 16 ) is also adapted to provide power signals to the network devices ( 18 ) via the power bus of the network bus ( 12 ). The network controller ( 16 ) and/or the bus protection elements can monitor signals on the network bus ( 12 ). And based upon the network controller ( 16 ) and/or bus protection elements identifying a predefined number of improper signals, the bus protection elements can selectively connect and disconnect respective network devices ( 18 ) to and from the network.

FIELD OF THE INVENTION

[0001] The present invention relates generally to systems and methodsfor reliably communicating over a network bus and, more particularly, tosystems and methods for maintaining the proper termination anderror-free communication in a network bus in the event of various typesof bus failures including a short circuit condition, an open circuitcondition or a network device that improperly occupies the network buswith meaningless data.

BACKGROUND OF THE INVENTION

[0002] As systems, such as multimedia entertainment, communications,process control systems and diagnostic systems utilized by theautomotive and aerospace industries, become more complex, a need arisesfor additional devices to communicate, either with each other or with acentral controller or the like. Historically, these systems includeddedicated wiring extending between the various devices in order tosupport communications therebetween. As systems have become moreintegrated and the communications requirements have been increased, theamount of dedicated wiring that would be required can quickly becomeexcessively large, both in terms of the space required for the wiringand the cost of the wiring and the attendant installation. Moreover, asthe amount of dedicated wiring increased, the overall complexity of thesystem also generally increased as well as the likelihood that someportion of the wiring might be damaged or broken during or followinginstallation.

[0003] As such, network busses have been developed to provide a commoncommunication path between a plurality of devices. In automotive andaerospace applications, for example, a network bus can be utilized tomonitor various components and to collect diagnostic and statusinformation. In this regard, diagnostic and status information relatingto the strain, acceleration, pressure and/or temperature to which thevarious components are subjected may be collected and analyzed. By wayof further example, a network bus architecture is currently beingdeveloped to support process control applications, as wellscommunications and the delivery of multimedia information to theoccupants of a vehicle, such as an automobile, minivan, sports utilityvehicle, aircraft, boat or the like. Advantageously, this network buswould transport the audio signals, including streaming audio signals,produced by one or more of a radio, a cassette tape player, a compactdisc player or the like to selected speakers or headphone jacksthroughout the vehicle. Similarly, the network bus may support voice anddata communications with a cellular telephone carried by an occupant ofthe vehicle, as well as communications with a laptop computer, ahandheld computing device or the like. Also, the network bus maytransmit video signals, including streaming video signals, from atelevision receiver, a videocassette recorder or other video source toone or more video monitors. In addition, the network bus may transmitsensor and actuator signals to and from devices such as drivetraindevices, passive restraint devices, crash avoidance devices,drive-by-wire devices, or the like.

[0004] In addition to the variety of devices that are connected to anetwork bus, one or more controllers are also generally connected to thenetwork bus for receiving data from the various devices and for sendingcommands to the devices. Among other things, these commands specify themanner in which the various devices are to function including the mannerin which the various devices are to transmit information over thenetwork bus. Additionally, the controller(s) can receive input from anoperator, such as an occupant of the vehicle. This input can include,for example, an indication of the source(s) of the signals to betransmitted over the network bus as well as the destination of thesignals.

[0005] Traditionally, networks of the type described above havetransmitted data in analog format. Unfortunately, analog signals aresusceptible to noise introduced into the signals during datatransmission. Given that many of the transmitted signals have a lowamplitude to start with, this noise can corrupt the signal and decreasethe signal to noise ratio to levels that cause loss of resolution in thesignal. Further, as many of these network devices are scattered somedistance from the controller, the electrical lines connecting thenetwork devices to the controller may be sufficiently long to causesignal degradation due to DC resistance in the wiring.

[0006] In light of these shortcomings, it would be advantageous toutilize digital networks. But, many conventional digital networks sufferfrom a variety of problems themselves. For example, many existingdigital networks operate according to complicated protocols whichrequire each network device to have a relatively high level processor,thereby increasing the cost of the network devices. Complicatedprotocols also introduce overhead into the messages on the bus that arenot necessary for data acquisition and control. This overhead canseverely limit the number of data samples that can be transmitted on thebus. These networks also have other problems. For example, theygenerally do not support both acquisition and control, and theytypically only support networks that extend over relatively shortlengths. Further, these networks typically have bulky network deviceinterfaces, slow network data rates and/or a low network device count.Additionally, many computer systems that include digital networks do notoperate in a time-deterministic manner. As such, these computer systemsgenerally lack the capability to schedule a trigger command to thenetwork components that repeats or is interpreted and executed with anyprecision timing.

[0007] Regardless of the digital or analog nature of network, thenetwork bus may be damaged during or following installation. In thisregard, the network bus typically consists of a plurality of conductorsor wires that may extend great lengths between the various controllersand network devices. Due to accidents or other unforeseen circumstances,one or more of the wires may be broken, thereby creating an opencircuit. Thus, components on one side of the open circuit will be unableto communicate via the broken conductor with components on the otherside of the open circuit. Additionally, signals transmitted over thebroken conductor will be reflected by the broken end of the conductordue to the characteristic impedance mismatch. The reflected signals willthen be returned along the conductor, thereby interfering, bothconstructively and destructively, with other signals being transmittedvia the conductor. While the components on one side of the open circuitmay be able to communicate at relatively low data rates, such as tenkilobits per second, reflected signals will generally prevent effectivecommunications between the components at higher data rates such as tenmegabits per second or the like. The destructive interference caused bythe reflected signals caused by the open circuit will render the signalson the broken conductor more susceptible to noise, thereby furtherlimiting effective communications.

[0008] In instances in which one or more conductors of the network busare broken, one of two different approaches has generally been taken.According to one approach, the network bus remains unrepaired for atleast some period of time and communications continue over the networkbus, albeit at a relatively slow data transfer rate that is selected soas not to be corrupted by the reflected signals. Since a number ofapplications require that communications be conducted via the networkbus at relatively high data transfer rates, the intentional slowing ofthe data over the network bus to reduce, if not negate, the deleteriousimpact of reflected signals may be inappropriate. Alternatively,communications via the network bus can be halted and a technician orrepair person can troubleshoot the network bus to identify the break inthe network bus and can then physically repair the broken conductors.Once the repairs have been completed, the communications over thenetwork bus can be recommenced. However, the physical repair of thenetwork bus oftentimes requires that the network bus be removed fromservice for some period of time, which action may also be inappropriatefor certain applications, such as time-sensitive applications or otherapplications that demand continuous monitoring or feedback.

[0009] A network bus may have other types of failures in addition tofailures attributable to an open circuit. For example, one or more ofthe pairs of conductors that comprise the network bus may develop ashort circuit. In this instance, the network controller and the networkdevices will no longer be able to communicate via the pairs ofconductors that are shorted. As such, the network bus would have to beremoved from service, the location of the short circuit would have to beidentified, and the network bus could be repaired, prior to recommencingcommunications via the network bus.

[0010] In addition to short circuit and open circuit conditions on thenetwork bus, a network device that is electrically connected to thenetwork bus may fail and, as a result, may create problems for thenetwork bus and for other network devices connected to the network bus.In this regard, some failure modes of the network devices areself-limiting in that failure of a network device does not adverselyimpact continued communications of the other network devices over thenetwork bus. However, other failure modes of the network devices maycreate problems on the network bus and effectively prevent other networkdevices from communicating via the network bus. One example of a failuremode of a network device that creates problems on the network bus isexhibited in instances in which a network device that fails emits astream of meaningless data onto the network bus. In this situation, thenetwork device is typically described to be “babbling.”

[0011] A network device that is babbling can monopolize control of thenetwork bus and can prevent the network controller and the other networkdevices from communicating via the network bus. In this situation, thenetwork device that is babbling must be identified and removed from thenetwork bus in order to permit the other network devices and the networkcontroller to communicate via the network bus. As will be apparent, theprocess of identifying the network device that is babbling andthereafter removing the babbling network device from the network bus canbe time-consuming, during which time the network bus will be unavailablefor communications between the other network devices and the networkcontroller.

[0012] Yet another type of failure of a network device that could causethe whole network to fail is that of a short circuit condition insidethe network device that causes it to draw too much current. If thenetwork device draws more than the amount of current that can properlybe supplied by the power supply in the network controller and powerconductors of the network bus, the power voltage supplied to the othernetwork devices will drop to levels that will cause them to beinoperable. As will be apparent, the process of identifying the networkdevice that is shorted and thereafter removing the shorted networkdevice from the network bus can be time-consuming, during which time thenetwork bus will be unavailable for communications between the othernetwork devices and the network controller.

[0013] As described above in conjunction with the repair of a networkbus having an open circuit condition, the removal of a network bus fromservice in order to repair a short circuit condition or to remove anetwork device that is babbling from the network bus will require thatthe network bus be out of service for some period of time and wouldtherefore be inappropriate for certain applications includingtime-sensitive applications or other applications that demand continuousmonitoring or feedback.

[0014] Accordingly, it would be advantageous to develop an improvednetwork bus that could accommodate bus failures caused by open and shortcircuit conditions and by network devices that begin babbling. Moreover,it would be desirable for the improved network bus to support continuedcommunications between the devices connected to the network bus withouthaving to slow the data transfer rate and without having to remove thenetwork bus from service in order to physically repair the network bus.

SUMMARY OF THE INVENTION

[0015] A system and method for maintaining the proper termination of anetwork bus are therefore provided. According to the present invention,the system and method can identify bus failure due to open or shortcircuit conditions or due to a network device that is babbling orshorted and can automatically reconfigure the network bus to continue tosupport communications without slowing the data transfer rate andwithout having to remove the network bus from service for any extendedperiod of time in order to physically repair the network bus. As such,the system and method of the present invention support continued highspeed data communications over the network bus even in the event of abus failure due to an open or short circuit condition or due to anetwork device that is babbling.

[0016] According to one aspect of the present invention, at least onenetwork device electrically connected to and adapted to communicate viathe network bus. The network devices are capable of transmitting signalsto, and receiving signals from, the network bus where the network busincludes a power bus. The system also includes a network controller thatis electrically connected to the network bus and is adapted to directcommunications with the network devices via the network bus. The networkcontroller is also adapted to provide power signals to the networkdevices via the power bus of the network bus.

[0017] To protect signals on the network bus, including the power bus,the system includes at least one bus protection element disposed betweenrespective network devices and the network bus. The network controllerand/or the bus protection elements can monitor signals on the networkbus, such as signals transmitted and received by the network devices tothereby identify improper signals. And based upon the network controllerand/or bus protection elements identifying a predefined number ofimproper signals, the bus protection elements can selectively connectand disconnect respective network devices to and from the network. Forexample, the bus protection elements can monitor the signals transmittedand received by the respective network device to identify impropersignals. If a predefined number of improper signals are identified, thebus protection element can then controllably halt the transmission ofsignals from the respective network device to the network bus

[0018] Each bus protection element can include a communicationsinterface having a transmitter for transmitting signals from the networkdevice to the network bus, and a receiver for receiving signals onbehalf of the network device from the network bus. To control thetransmission of signals from the network device to the network bus, thebus protection element includes at least one isolation switch capable ofbeing disposed in on and off modes and positioned between thecommunications interface and the network bus. Additionally, the busprotection element includes a logic element electrically connected tothe isolation switches and the communications interface. The logicelement is capable of monitoring the signals transmitted by thetransmitter and identifying improper signals. If a predefined number ofimproper signals are identified, the logic element can controllablyoperate the isolation switches to thereby halt the signals transmittedby the network device, such as by placing the isolation switches in theoff mode.

[0019] The bus protection element can further include a diagnosticswitch to allow the bus protection element to perform a diagnostic checkif the predefined number of improper signals are identified. Thediagnostic switch is disposed between the transmitter and the networkdevice, and can controllably halt the transmission of signals from thenetwork device to the transmitter. In this regard, the logic element cancontrollably operating the diagnostic switch if the predefined number ofimproper transmitted signals are identified. Also, the transmitter andreceiver can be electrically connected so that after the logic elementhalts the transmission of signals from the network device to thetransmitter and the transmission of signals from the transmitter to thenetwork bus, the logic element can transmit diagnostic signals throughthe transmitter and receive corresponding return signals from thereceiver. Knowing the diagnostic signals and receiving the returnsignals, the logic element can then compare the diagnostic signals andthe return signals and, if substantially identical, the logic elementwill determine that the network device is actually functioning properlyand can be reconnected to the network bus. Thus, the logic element cancontrollably operate the isolation and diagnostic switches to permit thetransmission of signals from the network device to the bus via thetransmitter, such as by operating the isolation and diagnostic switchesin the on mode.

[0020] According to another aspect of the present invention, each busprotection element includes a first set of switches in-line with thenetwork bus that is closed upon the application of power to the powerbus. By closing the first set of switches, the network device associatedwith the bus protection element can be connected to the network bus.Each bus protection element also includes a second set of switchesin-line with the network bus that are closed following the applicationof power to the power bus and in further response to a command from thenetwork controller. By closing the second set of switches, the networkcontroller can communicate, not only with the network device associatedwith the respective bus protection element, but also with other networkdevices connected to the network bus, downstream of the bus protectionelement. In addition, each bus protection element includes a terminationelement adapted to be switchably connected to the network bus upon theapplication of power to the power bus and to be switchably disconnectedfrom the network bus in response to a command from the networkcontroller. By connecting the termination element to the network buswhile the first set of switches is closed, but before the second set ofswitches is closed, the integrity of a portion of network bus can bemaintained. By disconnecting the termination element upon receipt of thecommand from the network controller, and closing the switches to thenext segment of the network bus, the network controller is permitted tocommunicate with other network devices downstream of the bus protectionelement. Typically, the same command issued by the network controllercauses the termination element to be disconnected and the second set ofswitches to be closed. The process of connecting the network devices tothe network then preferably continues, one at a time, until all of thenetwork devices are connected to the network bus.

[0021] In order to detect the power applied via the power bus, each busprotection element of this aspect of the present invention can include afirst power detector for detecting the application of power to the powerbus. If power is detected, the first power detector can signal the firstset of switches to close and can cause the termination element to beconnected to the network bus. Each bus protection element can alsoinclude a power switch in-line with the power bus and adapted to beclosed in response to the command issued by the network controller so asto supply power to the other network devices downstream of the busprotection element. Preferably, each bus protection element of thisaspect of the present invention includes a second power detector thatincludes the power switch for detecting the application of power to thepower bus and, if power is applied to the power bus, for signaling thesecond set of switches to close. By including the second power detector,the bus protection element can be symmetrical such that power andcommunications signals can be provided to the bus protection elementfrom either direction.

[0022] The termination element is preferably switchably connected to thenetwork bus between the first and second sets of switches in order toproperly terminate the network bus while the first set of switches isclosed and prior to closing the second set of switches. In this regard,each bus protection element preferably includes the same number oftermination elements as the number of communication pairs of conductorsat the network bus. Each termination element is therefore adapted to beswitchably connected across a respective pair of conductors. Inaddition, each termination element advantageously includes a resistorhaving a resistance that matches the characteristic impedance of thenetwork bus in order to properly terminate the network bus.

[0023] In one embodiment of this aspect of the present invention, thesystem includes first and second network controllers, connected toopposite ends of the network bus. Thus, in the event of a bus failure,each network controller can continue to communicate with a respectiveportion of the network bus even though the network bus is no longercontiguous.

[0024] In one embodiment, each network device is associated with anetwork device interface. The network device interface is connected tothe network bus and is separately connected to the network device via adata channel. In this embodiment, the network device interface caninclude the bus protection element.

[0025] A network protection element and method for maintaining propertermination and error-free communication conducted via a network busthat is electrically connected to at least one network device are alsoprovided. Thus, the present invention provides a system, networkprotection element and method that permit a bus failure to be identifiedand the network bus to be automatically reconfigured such that thefunctionality of the network bus will be maintained. As such,communications can continue over the network bus at a high data transferrate without concern for undesirable reflections or other noise causedby a bus failure such as an open or short circuit or the connection of ababbling or shorted network device. Moreover, the system and method ofthe present invention permit continued communications over the networkbus without having to remove the network bus from service in order tophysically repair the network bus. Thus, communications can continue andthe network bus can be scheduled for repair at some more convenienttime, such as during some period of time in which the network bus wouldotherwise have been idle.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Having thus described the invention in general terms, referencewill now be made to the accompanying drawings, which are not necessarilydrawn to scale, and wherein:

[0027]FIG. 1 is a schematic block diagram illustrating a network systemthat would benefit from the network bus protection afforded by oneembodiment of the present invention;

[0028]FIG. 2A is an exploded schematic block diagram illustratingvarious elements of the bus protection system, according to oneembodiment of the present invention;

[0029]FIG. 2B is an exploded schematic block diagram illustratingvarious elements of the bus protection system, according to anotherembodiment of the present invention;

[0030]FIG. 3A is a flow chart illustrating the method of protectingcommunications conducted via a network bus, according to one embodimentof the present invention;

[0031]FIG. 3B is a flow chart illustrating the steps in identifyingimproper signals according to one embodiment of the present invention;

[0032]FIG. 3C is a flow chart illustrating diagnostic steps of themethod of protecting communications via a network bus, according to oneembodiment of the present invention;

[0033]FIG. 4A is an exploded schematic block diagram illustratingvarious elements of the bus protection element of another embodimentoperating with a network having separate pairs of conductors for thetransmitter and receiver;

[0034]FIG. 4B is an exploded schematic block diagram illustratingvarious elements of the bus protection element of yet another embodimentoperating with a network having separate pairs of conductors for thetransmitter and receiver;

[0035]FIG. 5 is a schematic representation of a system according to yetanother embodiment of the present invention that includes a pair ofnetwork controllers connected to opposite ends of a network bus;

[0036]FIG. 6 is a schematic representation of the transmission andreception portions of the first and second network controllers accordingto one embodiment of the present invention;

[0037]FIG. 7 is a schematic representation of a bus protection elementaccording to one embodiment of the present invention;

[0038]FIGS. 8A and 8B are schematic representations of two embodimentsof a power detector of a bus protection element according to the presentinvention; and

[0039]FIG. 9 is a schematic representation of the system of FIG. 5following a bus failure.

DETAILED DESCRIPTION OF THE INVENTION

[0040] The present invention now will be described more fullyhereinafter with reference to the accompanying drawings, in whichpreferred embodiments of the invention are shown. This invention may,however, be embodied in many different forms and should not be construedas limited to the embodiments set forth herein; rather, theseembodiments are provided so that this disclosure will be thorough andcomplete, and will fully convey the scope of the invention to thoseskilled in the art. Like numbers refer to like elements throughout.

[0041] Referring now to FIG. 1, a system 10 that would benefit from theprotection afforded by the present invention is depicted. While severalembodiments of the system are illustrated and will be hereinafterdescribed for purposes of example, other types of systems can readilyemploy the protection afforded by the present invention. Moreover, thesystem and method of the present invention will be primarily describedin conjunction with automotive and aerospace applications, such asautomotive and aerospace applications designed to support communicationsand the delivery of multimedia information to the occupants of a vehicleand/or to monitor various components, to collect diagnostic and statusinformation and provide process control information to sensors andactuators. However, the system and method of the present invention canbe utilized in conjunction with a variety of other applications, both inthe automotive and aerospace industries and outside of those industries.

[0042] As shown in FIG. 1, the system 10 includes a network bus 12electrically connected to a network controller 14. Whereas, the systemillustrated only includes a single network controller, it should beunderstood that more than one network controller can be utilized, suchas for fault tolerance purposes, without departing from the spirit andscope of the present invention. The system depicted in FIG. 1 alsoincludes a plurality of network devices 18 electrically connected to thenetwork bus and, in turn, to the network controller, at different pointsalong the network bus. Thus, the network bus supports communicationsbetween the network controller and the network devices, as well ascommunications between the network devices themselves.

[0043] The network controller 14 is designed to issue commands torespective network devices 18 and, in at least some instances, toreceive data from the network devices. For example, the commands maydictate that a network device provide a particular type of data, such asstatus or other diagnostic data. Alternatively, the commands issued bythe network controller can direct one or more of the network devices toprovide signals of a predetermined type, such as audio signals, videosignals or the like, and for one or more of the other network devices toreceive the signals. Although various types of network controllers canbe utilized, one advantageous type of network controller is the networkcontroller described by U.S. patent application Ser. No. 09/736,878entitled Network Controller for Digitally Controlling Network DevicesVia a Common Bus, filed Dec. 14, 2000, the contents of which areincorporated herein by reference. As described in U.S. patentapplication Ser. No. 09/736,878 and as illustrated in FIG. 1, thenetwork controller may also be disposed in electrical communication witha host computer 20 that generally directs the operation of the networkcontroller and that analyzes data received by the network controllerfrom the network devices.

[0044] Various types of network buses 12 can be employed. Typically, thenetwork bus consists of one, two, three or more pairs of wires, such asdifferential twisted pair copper wires, for transmitting commands, dataand other signals. As such, the network bus can be designed to supporteither a standard half duplex configuration or a full duplexconfiguration. In some embodiments desiring synchronous communications,one of the pair of wires can be utilized to transmit a clock signaltypically from a network controller to the network devices 18. Stillfurther, the network bus can include a pair of power lines, such aspower wires, for supplying power and a common ground to the remotedevices.

[0045] The network devices 18 include, and are associated with,respective remote devices that can include a wide variety of devices;most, if not all, of which are located remote from the networkcontroller 14. For example, the remote devices can include sensors forproviding data, such as status or diagnostic data, to the networkcontroller so that the health and/or operation of whatever the sensor issensing can be monitored. In an automotive application, for example, theremote devices can include sensors for monitoring the throttle position,the oil pressure, the water temperature, the transmission fluidpressure, the seat position, the antilock brake system, the suspension,the passive restraint system and the steering system, to name but a few.Alternatively, the remote devices may include an actuator for performinga particular function in response to a command from the networkcontroller. In the automotive application, for example, the remotedevices can include actuators for controlling the throttle position, theanti-lock brake system, the suspension, the passive restraint system andthe active suspension system, to name but a few. Still further, theremote devices may include an audio or video source. In this regard, theremote devices can include radio receivers, tape cassette players,compact disc players, and cellular telephone receivers for providingaudio signals, including in some instances streaming audio signals, tothe network bus. Correspondingly, the remote devices can includespeakers, headphone jacks or the like for receiving audio signals fromthe network bus and for providing a corresponding audible output.Similarly, the remote devices can include a television receiver or avideo cassette player providing video signals to the network bus.Accordingly, the remote devices can include a video monitor or the likefor receiving the video signals and for displaying images based upon thevideo signals.

[0046] While the remote devices 19 of the respective network devices 18may be directly connected to the network bus 12, the network devices 18of one advantageous embodiment include respective network deviceinterfaces 16 as described by U.S. patent application Ser. No.09/735,146 entitled Network Device Interface for Digitally InterfacingData Channels to a Controller Via a Network, filed Dec. 12, 2000, thecontents of which are incorporated herein in their entirety. In thisadvantageous embodiment, the network device interface is disposed inelectrical communication with the network bus for transmitting signalsto the network bus and for receiving signals from the network bus. Eachnetwork device interface is also in communication with one or moreremote devices via respective data channels. While each network deviceinterface is shown to be connected to a single remote device in FIG. 1,the network device interface could be connected to two or more remotedevices, if so desired.

[0047] As described by U.S. patent application Ser. No. 09/735,146, thenetwork device interface 16 performs various functions to facilitatecommunications by the remote device 19 over the network bus 12. Forexample, the network device interface may store the data collected bythe associated remote device(s) such that the stored data may beprovided to the network controller 14 via the network bus upon request.If the remote device is an analog device, the network device interfacemay also convert signals between the digital format supported by thenetwork bus and the analog format supported by the remote device. Forpurposes of explanation and not of limitation, however, the presentinvention will subsequently be described in terms of the networkdevices, which include respective remote devices and can additionallyinclude associated network device interfaces.

[0048] Upon initialization of the network, the network controller 14inventories the network devices 18 connected to the network bus 12 andassigns a unique logical address to each network device so that thenetwork controller can communicate with a specific network device or aspecific group of network devices. A wide variety of techniques can beutilized to inventory the network devices connected to the network busand to assign unique logical addresses to the network devices. Oneadvantageous technique for inventorying the network devices andassigning unique logical addresses is the bit competition techniquedescribed by U.S. patent application Ser. No. 09/736,878, U.S. patentapplication Ser. No. 09/735,146 as well as Provisional U.S. PatentApplication No. 60/286,793 entitled Systems and Methods for Assigning anAddress to a NetworkDevice Added to an Existing Network, filed Apr. 26,2001, and PCT Patent Application No. ______ entitled Systems and Methodsfor Assigning an Address to a Network Device Added to an ExistingNetwork, filed concurrently herewith, the contents of all of which arealso incorporated herein in their entirety.

[0049] In operation, the network controller 14 issues various commandsand respective network devices 18 respond based upon the commands. Thenetwork controller and the network devices can communicate according toany of a number of different protocols. As described in U.S. patentapplication Ser. No. 09/736,878, for example, the network controller andthe network devices can communicate according to a Manchester-encodedbi-phase sensor and system (BiSenSys) protocol. Alternatively, thenetwork controller and the network devices can communicate according toa protocol compatible with a universal asynchronous receiver transmitter(UART) physical layer. Preferably, however, the protocol is selected tominimize the overhead and to correspondingly maximize the data transfercapability. Further, the protocol is preferably selected to relativelysimple such that neither the network devices nor the network deviceinterfaces 18 will require a high level processor. Instead, the networkcontroller and the associated host computer 20 can include the majorityof the processing power and the network device interfaces can includelogic that is readily implemented in hardware, software or firmware. Thecommunications supported by the system 10 may also be either synchronousor asynchronous and may involve the transmission of various types ofmessages. As described by U.S. patent application Ser. No. 09/736,878,for example, one advantageous communications technique is based upon thetransmission of message frames that include command frames and dataframes having respective predetermined lengths or sizes. Depending uponthe protocol, the system can also support various command sets. As withthe protocol, the command set is preferably selected to minimize theoverhead that must be transmitted via the network bus and to berelatively simple. One example of a suitable command set is described byU.S. patent application Ser. No. 09/735,146.

[0050] Regardless of the protocol implemented by the system 10, thenetwork controller 14 can issue commands via the network bus 12 andthereafter await a response from the network devices 18. By way ofexample, the following table illustrates a command designated Triggerissued by the first network controller that polls specific networkdevices for data that has been collected by the network devices. Theinitial trigger command can poll the network devices designated S1, S2and S5, while a subsequent trigger command polls the network devicesdesignated S1, S3, S4, S6, S7 and S8 (shown in FIG. 5 in conjunctionwith an aspect of the present invention including two networkcontrollers). As indicated, the network devices can be polled atdifferent intervals, as demonstrated by the more frequent polling of thenetwork device designated S1. First Network Controller Second NetworkController Trigger Poll S1 Poll S2 Poll S5 Trigger Poll S1 Poll S3 PollS4 Poll S6 Poll S7 Poll S8

[0051] As previously stated, it is anticipated that all network deviceswill fail at some point. To that end, referring now to FIG. 2A, thesystem 10 includes at least one bus protection element to protect thenetwork bus 12 (and the system) from malfunctioning network devices(e.g., babbling idiots) that would otherwise result in a loss of thenetwork bus. Each network device includes a communications interfacehaving a transmitter 22 for transmitting signals from a respectivenetwork device 18, and a receiver 24 for receiving signals on behalf ofthe network device from the network bus. The transmitter and receivercan comprise any of a number of different elements, as such are known.Additionally, whereas the transmitter and receiver can be separateelements, as illustrated, a single transceiver can be used to performthe functions of both the transmitter and the receiver.

[0052] Typically, the network device 18 transmits signals to the networkbus 12 and receives signals from the network bus through the transmitter22 and receiver 24, respectively. To control the transmission of signalsfrom the network device to the network bus 12 and the reception ofsignals to the network device from the network bus, the bus protectionelement includes at least one isolation switch 26 disposed between thetransmitter and the receiver and the network bus. Further, to controlthe transmission of signals from the network device to the network buswithout controlling the reception of signals to the network device, thebus protection element includes at least one separation switch 27. Theisolation switches act to controllably halt the transmission of signalsfrom the network device onto the network bus and the reception ofsignals to the network device from the network bus. And the isolationswitches and/or separation switches can controllably halt thetransmission of signals from the network device onto the network buswithout preventing the network device from receiving signals, such as aReset command, from the network bus. Whereas the isolation switches andseparation switches can comprise any of a number of different elements,in a preferred embodiment the isolation switches comprise field effecttransistors (FETs). Typically, the isolation switches and separationswitches operate in either an on mode wherein the isolation switches andseparation switches permit the network device to transmit signals to thebus. In contrast, the isolation switches and separation switches canoperate in an off mode where, if either is operating in the off mode,the respective switches prevent the network device from transmittingsignals to the bus. Further, if the isolation switches are operating inthe off mode, the isolation switches prevent the network device fromreceiving signals from the bus. In this regard, FETs provide a very highimpedance when in the off mode to thereby halt the transmission ofsignals, and a very low impedance when in the on mode to thereby permitthe transmission of signals. For another embodiment of the busprotection element, see FIG. 2B.

[0053] The bus protection element also includes a logic element 28electrically connected to the isolation switches 26, the transmitter 22and the receiver 24. The logic element can comprise any number ofdifferent components and can be comprised of hardware, software orfirmware, but typically comprises a state machine implemented as anapplication specific integrated circuit (ASIC) or a field programmablegate array (FPGA). The logic element is preferably included within thenetwork device interface 16 associated with the network device 18, butthe logic element can be a separate element. Regardless of whether thelogic element is included within the network device element or aseparate element, the logic element can be electrically isolated fromthe components of the network device interface and from the networkdevice, such as in an isolated section of the network device interfaceor a separate element isolated from the network device interface and thenetwork device. In this regard, the logic element will not be affectedby any failure in the network device interface or the network device.

[0054] The logic element 28 is capable of monitoring the signalstransmitted by the network device 18 and identifying improper signals.The logic element can identify any of a number of various impropersignals but, in a preferred embodiment, the bus protection elementidentifies the signals transmitted as improper if the signals consist ofimproper signals transmitted independent of signals received by thenetwork device (e.g., signals transmitted that are not in response to acommand from the network controller 14), improperly formatted signalstransmitted by the network device, improper signals transmitted inresponse to signals received by the network device, and/or improperlylong single-state signals transmitted by the network device (e.g., animproperly long logic one state or logic zero state), as describedbelow.

[0055] If the logic element 28 identifies a predefined number ofimproper signals, the logic element can controllably operate theisolation switches 26 and/or the separation switches 27 to thereby haltsignals transmitted by the network device 18, typically by placing theisolation switches an/or separation switches in the off mode. In orderto be somewhat fault tolerant, the logic element can allow a predefinednumber of improper signals to pass to the network bus 12 beforeisolating the respective network device from the network bus, such asallowing three improper signals before placing the isolation switches inthe off mode. Allowing for the predefined number of improper signalsprevents the logic element from unnecessarily isolating the respectivenetwork device from the network if an improper signal is infrequentlytransmitted from the network device. Additionally, or alternatively, thepredefined number of improper signals can be dependent upon apredetermined number of signals transmitted by the network device. Forexample, the logic element can allow the network device to transmitthree improper signals for every one hundred signals transmitted to thenetwork bus.

[0056] Although not required for the system 10 to protect thecommunications conducted via the network bus 12, the bus protectionelement can further include diagnostic switches 30, disposed between thetransmitter 22 and the network device 18, that allows the logic element28 to perform a diagnostic check if the predefined number of impropersignals are identified. The diagnostic switches can controllably haltthe transmission of signals from the network device to the transmitter,and typically operate in either an on mode during normal operation ofthe system wherein the diagnostic switches permit the respective networkdevice to transmit signals to the transmitter, or an off mode whereinthe diagnostic switches prevent the respective network device fromtransmitting signals to the transmitter. The diagnostic switches cancomprise any of a number of elements but, in a preferred embodiment,comprise FETs.

[0057] If the logic element 28 identifies the predefined number ofimproper transmitted signals, the logic element can operate thediagnostic switches 30, as well as operate the isolation switches 26,typically by placing the diagnostic switches and the isolation switchesin the off mode. The logic element can operate the diagnostic switchesto perform the diagnostic check after identifying the predefined numberof any of the various improper signals but, in a preferred embodiment,the bus protection element operates the diagnostic switches to performthe diagnostic check after identifying the predefined number ofimproperly long single-state signals transmitted by the network device18.

[0058] After the logic element operates the diagnostic switches and theisolation switches, the logic element can perform the diagnostic checkto determine whether the respective network device or respectivetransmitter 22 are malfunctioning or whether another network device ortransmitter on the network is malfunctioning, as described below. Inother words, the diagnostic check determines if the network device orthe transmitter that has been isolated from the network bus aremalfunctioning, or if the improper signals transmitted by the respectivenetwork device are really the product of another network device sincethe respective network device functions normally once removed from thenetwork bus. And if another network device or transmitter ismalfunctioning, then their respective bus protection element should actto protect the network bus 12 from the respective improper signals sincethe system 10 preferably contemplates that a bus protection element willbe associated with each network device.

[0059] Referring now to FIGS. 3A and 3B, upon initialization of thesystem 10, the isolation switches 26, separation switches 27 anddiagnostic switches 30 operate to allow the network device 18 totransmit signals to the transmitter 22, and allow the transmitter totransmit signals to the network bus 12 (block 40). As the transmittertransmits signals to the network bus, the logic element monitors thesignals to thereby identify the improper signals (blocks 42, 44). Toidentify improper signals transmitted independent of signals received bythe network device, the logic element 28 monitors the receiver 24 andthe transmitter 22 to ensure the signals transmitted are preceded bysignals received by the receiver (FIG. 3B, block 50). In this regard,the network devices are configured to transmit signals only afterreceiving a command signal, such as from the network controller 14.Therefore, the logic element monitors the receiver for a signal and,once a signal has been received, determines whether the signal is acommand signal expecting a response from the network device (blocks 52,54). If received signal is not a command signal expecting a response,any signal transmitted by the remote device is an improper signal (block56).

[0060] To identify improperly formatted signals transmitted by thenetwork device 18, the logic element 28 identifies and interprets thesignals transmitted to compare the format of the signals transmittedwith the format of the protocol utilized by the network controller 14and network devices to communicate, such as the BiSenSys or UARTcompatible protocols described by U.S. patent application Ser. No.09/736,878 (block 58). For example, the BiSenSys protocol consists ofeither a command sync pulse (including one and one half bit length highfollowed by one and one half bit length low) or data sync pulse(including one and one half bit length low followed by one and one halfbit length high), plus a 17 bi-phase bit argument and a bi-phase paritybit.

[0061] The logic element 28 identifies improper signals transmitted inresponse to command signals received by the network device 18 bymonitoring the signals received by the receiver 24. After the logicelement has identified and interpreted the received signals according tothe appropriate protocol, the logic element determines if the signalstransmitted are proper based upon the interpreted signals received bythe receiver and further based upon applicable protocol standards.

[0062] In order to identify proper signals based upon applicableprotocol standards, the logic element 28, can include diagnostic logicof various sophistications. In this regard, the logic element canmonitor the incoming command signals according to a method such as thatdisclosed in U.S. patent application Ser. No. 09/736,878. The methoddisclosed in U.S. patent application Ser. No. 09/736,878 includes asmall instruction set particularly suited for miniature embedded logicsystems. By incorporating such a small instruction set, the diagnosticlogic required to monitor for proper responses to specific commands isdiminutive. As previously described, the logic element can identifyimproper signals by detecting improper signals transmitted without anaccompanying received signal (blocks 52, 54). Additionally, oralternatively, the logic element can identify improper signals bydetecting improperly formatted signals from the network device 18 inresponse to an appropriate command signal (block 60), as well as theimproper use of a parity bit within the transmitted signal (block 62).

[0063] Also, the logic element 28 can identify improper signals bydetermining if the signal transmitted includes the proper number of datawords (blocks 64, 66), or if the transmit enable line has been properlyreleased once transmission of a response signal has been completed(block 68). For example, if the network device 18 received a ReadIn-Data Word command signal, the logic element would look for thenetwork device to transmit a one word signal. Since the logic elementmay not be sophisticated enough to know the address of the networkdevice the logic element is protecting, the logic element will not knowif the network device the logic element is monitoring will transmit aresponse signal. But if the network device does transmit a responsesignal, the network device should only respond with a one word responsesignal in the proper format.

[0064] Another improper signal identified by the logic element 28,improperly long single-state signals transmitted by the network device18 (e.g., an excessively long dormant or recessive state identified onthe network bus 12), indicates that the transmitter 22 may be stuck in adormant state due to either a stuck enable line from the network device(or, more particularly, the network device interface 16) or amalfunction in the transmitter. To identify improperly long single-statesignals transmitted by the network device, the logic element monitorsthe signals received by the receiver 24 for a single-state signal longerthan that allowed by the logic element. In this regard, the network ofsome embodiments is an actively terminated network such that when nosignals are present on the network bus, the network bus is pulled into abiased state, i.e., a tri-state, that can be interpreted by the logicelement as an excessively long single-state signal received by thereceiver. Thus, to identify a single-state signal as improper as opposedto a mere lack of signals on the network bus, the network controller 14may have to be configured to issue a continuous stream of signals tomaintain traffic on the network bus. As such, a stuck enable line or amalfunctioning transmitter can be identified by the lack of signals onthe network bus.

[0065] After identifying a transmitted signal as improper, the logicelement 28 will determine if the predefined number of improper signalshave been identified and, if so, the logic element will operate theisolation switches 26 to protect the network bus 12 from furtherimproper signals transmitted by the transmitter 22 to the network bus,typically by placing the isolation switches in the off mode (FIG. 3A,blocks 46, 48). After the logic element places the isolation switches inthe off mode, the logic element can perform a diagnostic check of thetransmitter and receiver 24, as described below. If no diagnostic checkis to be performed, however, instead of operating the isolation switchesin the off mode, the logic element will generally operate separationswitches 27 to isolate the transmitter of the respective network device18 from the network bus while still allowing the receiver 24 of therespective network device to receive signals. The transmitter ofrespective network device will typically remain unable to transmitsignals to the network bus until a reset condition is executed in thenetwork, such as a Reset command sent from the network controller 14 orthe network device completing a power cycle. In addition to the resetcondition allowing the network device to transmit signals to the networkbus, the logic element can operate the isolation switches to allow thetransmission of signals from the network device after performing adiagnostic check and determining that the respective network device is,in fact, functioning properly, such as is described below.

[0066] Referring now to FIG. 2A and FIG. 3C, if so desired, the logicelement 28 can operate the diagnostic switches 30 to perform adiagnostic check of the transmitter 22 and receiver 24 after identifyingthe predefined number of improper transmitted signals and, mostcommonly, after identifying the predefined number of improperly longsingle-state signals transmitted by the network device 18. In thisregard, after the logic element has operated the isolation switches 26,the logic element will operate the diagnostic switches, such as byplacing the diagnostic switches in the off mode, to thereby prevent therespective network device from transmitting signals to the transmitter22 (block 70).

[0067] After the logic element 28 isolates the transmitter from thenetwork device 18, the logic element transmits a set of diagnosticsignals to the transmitter 22, such as a known pattern of signals (block72). Once the diagnostic signals have been transmitted to thetransmitter, the logic element receives corresponding return signalsfrom the receiver 24 (block 74). In this regard, the transmitter and thereceiver are in electrical communication, such as in the case of thenetwork bus 12 operating in a half-duplex mode.

[0068] Referring to FIG. 4A, some network bus 12 configurations includeseparate pairs of conductors for the transmitter 22 and the receiver 24,such as when the network bus is operating in a full-duplex mode. Inembodiments wherein the network bus includes separate pairs ofconductors, separation switches 27 are connected to the pair ofconductors connected to the transmitter to thereby halt the transmissionof signals from the respective network device to the network bus whenthe predefined number of improper signals are identified. But to performthe diagnostic check after halting the transmission of signals, the busprotection element also includes the isolation switches 26 connected tothe receiver to isolate the receiver from the network bus and, thus,allow the receiver to receive signals from the transmitter during thediagnostic check without interference from the signals on the networkbus. Additionally, because the transmitter and receiver are not inelectrical communication during normal operation, the bus protectionelement of this embodiment includes a pair of connecting switches 32 toplace the transmitter and receiver in electrical communication duringthe diagnostic check. The connecting switches can consist of a varietyof different types of switches but, preferably, consist of FETs.

[0069] Regardless of the configuration of the bus protection element,once the logic element 28 has received the corresponding return signals,the logic element can compare the return signals to the known diagnosticsignals (block 76). If the return signals are substantially differentthan the diagnostic signals, the logic element can operate theseparation switches 27 in the off mode and operate the isolationswitches 26 in the on mode, while continuing to operate the diagnosticswitch 30 in the off mode (block 77). As such, the logic element canprevent the transmitter 22 from transmitting signals to the network bus12 while allowing the receiver 24 to receive signals, such as a resetcommand, from the network bus. In this regard, the difference in thesignals is indicative of a malfunction in the network device 18 and/orthe transmitter 22. As such, the logic element performs the diagnosticcheck to determine whether its respective network device 18 ortransmitter 22 are malfunctioning, or whether another network device ortransmitter on the network are malfunctioning and causing the respectivenetwork device to issue seemingly improper signals. If the respectivenetwork device and transmitter are not malfunctioning, the returnsignals and diagnostic signals will be substantially identical, and thelogic element will operate the isolation and diagnostic switches tothereby permit the respective network device to transmit signals to thenetwork bus 12, such as by placing the isolation and diagnostic switchesin the on mode (block 78). In this regard, the bus protection elementassociated with the malfunctioning network device or transmitter willact to protect the network bus from the improper signals. For anotherembodiment of the bus protection element where the network bus includesseparate pairs of conductors, see FIG. 4B.

[0070] According to aspect of the present invention, shown in FIG. 5,the system 10 includes a network bus 12 that extends between a pair ofnetwork controllers. Thus, a first network controller 14 a iselectrically connected to a first end of the network bus, while a secondnetwork controller 14 b is electrically connected to the second end ofthe network bus. In the illustrated embodiment, the first and secondnetwork controllers are co-located such that the network bus forms aloop between the pair of network controllers. The two networkcontrollers 14 a and 14 b are shown as separate devices for purposes ofillustration, but it should be understood that the two networkcontrollers can be embodied in a single device. Also, as will beapparent, whereas the network controllers are illustrated as located atends of the network bus, the network controllers can be located atdifferent positions from one another, if so desired. Irrespective ofwhether the network controllers are embodied in a single device andwhether the network controllers are located at ends of the network bus,the two network controllers are preferably connected to each other sothat the network controllers have a dedicated communication link witheach other. The network controllers can be connected to each otheraccording to any of a number of different known methods, including alink 79 through the host computer 20 or a separate link 81 between thenetwork controllers.

[0071] During ordinary operation of the embodiment of the system 10depicted in FIG. 5, the first network controller 14 a typically servesas a master controller and is responsible for issuing commands over thenetwork bus 12. In contrast, the second network controller 14 b isgenerally a slave controller that monitors the network bus so as todetect the various commands issued by the first network controller andthe responses provided by the network devices 18. The second networkcontroller is preferably capable of issuing commands and the secondnetwork controller may actually be required to assume control of aportion of the network bus in the event of a bus failure, as describedhereinbelow.

[0072] In order to prevent undesirable reflections of the signalstransmitted via the network bus 12, the opposed ends of the network busare preferably properly terminated. In this regard, the ends of thenetwork bus are preferably terminated by a termination element having animpedance that matches the characteristic impedance of the network bus.In one embodiment in which the network bus consists of several pairs ofwires, a respective termination element is placed across each pair ofwires at each of the opposed ends of the network bus. The characteristicimpedance of the network bus is typically resistive, such that thetermination element is also typically a resistor. By way of example, thenetwork bus generally has a characteristic impedance of about 100-120ohms, with a Category 5 Ethernet bus having a characteristic impedanceof 100 ohms. As such, each resistor placed across a pair of wirespreferably has the same resistance in order to appropriately terminatethe network bus and prevent undesirable reflections.

[0073] Where the network controllers 14 a, 14 b are connected toopposite ends of the network bus 12, the network controllers preferablyinclude the termination elements. As depicted in FIG. 6, for example,the network controllers can be designed to communicate with a networkbus that includes a power bus (designated power + and power −) and threepairs of conductors (designated pairs 1, 2 and 3). As such, the networkbus of this illustrated embodiment is designed for full duplexcommunications over the first and second pairs of conductors. A clocksignal can also be provided, if necessary, via the third pair ofconductors. While the system 10 can be configured in a variety ofdifferent manners without departing from the spirit and scope of thepresent invention, the first network controller serves as the mastercontroller in accordance with one embodiment and therefore includes atransmitter 82 connected to the first pair of conductors fortransmitting commands via the first pair of conductors. In order toreceive the responses from the network devices 18, the first networkcontroller also includes a receiver 84 connected to the second pair ofconductors. In addition, the first network controller of this embodimentincludes a transmitter 86 connected to the third pair of conductors fortransmitting a clock signal, if necessary. In this embodiment, thesecond network controller is a slave controller that is primarilydesigned to receive the signals transmitted by the first networkcontroller and the network devices. Thus, the second network controllerincludes receivers 88 connected to each of the three pairs ofconductors. Since the second network controller is designed, however, tocontrol at least a portion of a network bus in the event of a busfailure as described in detail below, the second network controller ofthis embodiment also includes a transmitter 90 connected to the firstpair of conductors for transmitting commands via the first pair ofconductors, and a transmitter 92 connected to the third pair ofconductors for transmitting a clock signal, if necessary. In order toproperly terminate each pair of conductors, the termination elements 80,such as a resistor having a resistance that matches the characteristicresistance of the network bus, are connected across the pairs ofconductors, as shown in FIG. 6. The termination elements thereforeprevent undesirable reflections.

[0074] According to this aspect of the present invention, a busprotection element 94 is preferably associated with each network device18. For example, the bus protection element may be integral to arespective network device. In one advantageous embodiment, however, anetwork device interface 22 is associated with each network device andincludes the bus protection element. It should be understood, however,that the bus protection element can be independent of a network deviceinterface, if so desired. Regardless of its implementation, each busprotection element is adapted to selectively connect at least onenetwork device to the network bus 12. Although a bus protection elementcan be configured in a number of different manners, one advantageousembodiment of a bus protection element is depicted in FIG. 7 and isdescribed in detail hereinafter. In this embodiment, the bus protectionelement is disposed in electrical connection to the network bus. Inparticular, the bus protection element is disposed in electricalconnection with the power bus and with each pair of conductors of thenetwork bus.

[0075] As described below, each bus protection element 94 of this aspectof the present invention includes a number of switches for connectingthe associated network device 18 to the network bus 12. Prior to powerbeing applied to the system 10, the nominal position of each of theswitches of the bus protection element is in the open position. As such,the network devices with which the bus protection elements areassociated will not be connected to the network bus. Upon initializationof the system 10, the network controllers 14 a, 14 b apply power via thepower bus. In the embodiment of FIG. 5 in which the system includesfirst and second network controllers, power is preferably applied byboth the first and second network controllers from opposite ends of thenetwork bus.

[0076] Each bus protection element 94 includes a power detector 96 fordetecting the application of power to the power bus. While a variety ofpower detectors can be employed, one embodiment of a power detector isdepicted in FIG. 8A. In this embodiment, the power detector includes amechanical relay that is normally open. Upon the application of power tothe power bus, the mechanical relay will close, thereby providing powerto the network device(s) 18 associated with the bus protection element.It will be understood, however, that the bus protection element caninclude other types of power detectors and switches. For example, thepower detector can be designed to include a charge pump and a solidstate switch, such as an n-channel MOSFET, as shown in FIG. 8B.Regardless of the type of power detector, the bus protection elementtypically includes a power switch 110 in-line with the power bus andcontrolled by the first power detector so as to be closed upon thedetection of power by the power detector.

[0077] As shown in FIG. 7, the bus protection element 94 of thisembodiment also includes a first set of switches 98 in-line with eachconductor of the network bus, other than the power bus. The first set ofswitches are also normally open. However, the first set of switches arecontrolled by the power detector 96 so as to be closed once the powerdetector detects that power has been applied to the power bus and beginsto conduct. Since the network device 18 associated with the busprotection element is connected to the network bus downstream of thefirst set of switches, the closure of the first set of switches permitsthe network device to receive commands and clock signals from thenetwork controller and to transmit responses or data in reply thereto.In the full duplex configuration depicted in FIG. 7, for example, thenetwork device is connected to the first and third pairs of conductorsby means of first and second receivers 100 for receiving commands andclock signals, respectively. Similarly, the network device is connectedto the second pair of conductors by a transmitter 102 for transmittingresponses or data via the second pair of conductors to the networkcontroller or to other network devices.

[0078] While the network device 18 can be connected directly to thenetwork bus 12, the bus protection element 94 of the illustratedembodiment includes a logic element 103 for performing the variousfunctions of the bus protection element as described below. Moreover,the logic element of the bus protection element of the illustrateddevice is designed to receive signals from and transmit signals to thenetwork bus as also described below. Thus, the receivers 100 and thetransmitter 102 are connected to the logic element which, in turn, isconnected to the network device. Additionally, the logic element of thisembodiment may also serve to connect the network device to the powerbus. It should be understood that while FIG. 7 depicts the logic elementin a hardware block diagram, the logic element can also be comprised ofsoftware or firmware.

[0079] According to this aspect of the present invention, each busprotection element 94 also includes a termination element 104 switchablyconnected across the network bus 12 and, in particular, across each pairof conductors of the network bus, other than the power bus. As such, thebus protection element preferably includes the same number oftermination elements as the number of pairs of conductors. In theembodiment depicted in FIG. 7, for example, the bus protection elementpreferably includes three termination elements that can be switchablyconnected across the first, second and third pairs of conductors,respectively.

[0080] As shown in FIG. 7, each termination element 104 preferablyextends across a pair of conductors in series with a normally openswitch 106 such that the termination element is not normally connectedacross the network bus. Upon the application of power to the power busand the detection of power by the power detector 96, however, the powerdetector will direct the switches associated with each of thetermination elements to close, thereby placing the termination elementsacross respective pairs of the conductors. While the power detectorcould directly control the switches associated with the terminationelements, the bus protection element of the illustrated embodimentdirects the output of the power detector to the logic element 103 which,in turn, issues a signal causing the switches associated with each ofthe termination elements to close. Since the switches associated withthe termination elements are not directly connected to a specific powerdetector, the switches can be responsive to either power detector, viathe logic element, to facilitate the symmetrical operation of the busprotection element as described below. As described above, eachtermination element preferably has an impedance that matches thecharacteristic impedance of the network bus 12 in order to preventreflections. Since the characteristic impedance of the network bus istypically resistive, the termination element is typically a resistor asshown in FIG. 7. While each termination element is commonly placedacross a respective pair of conductors as shown in FIG. 7, thetermination element may be an active termination element as known tothose skilled in the art, if so desired.

[0081] By closing the first set of switches 98 to connect the networkdevice(s) 18 associated with the bus protection element 94 to thenetwork bus 12 concurrent with the switchable connection of thetermination elements 104 across the respective pairs of conductors ofthe network bus, the system 10 of the present invention and, moreparticularly, the bus protection element permits the associated networkdevice(s) to communicate via the network bus while reducing, if noteliminating, undesirable reflections.

[0082] During the power-up sequence prior to commencing operations, themaster controller will issue a command, typically termed “Identify,”requesting that the respective network device 18 to which power is beingapplied provide a response identifying itself. Once the power detector96 of a bus protection element 94 has detected that power has beenapplied to the power bus and has closed the first set of switches 98 andswitchably connected the termination elements 104 to the respectivepairs of conductors, the network device and, more commonly, the networkdevice interface 22 or the logic element 103 of the bus protectionelement in embodiments that do not include a network device interfacewill transmit a response to the network controllers identifying therespective network device.

[0083] Upon receiving the response identifying the network device 18,the master controller will generally issue a command termed “Connect”informing the bus protection element 94 associated with the currentnetwork device that the system 10 will attempt to connect to the nextnetwork device on the network bus 12. In response to the Connectcommand, the bus protection element will close a second set of switches108 that are also in-line with each conductor of the network bus, otherthan the power bus. As shown in FIG. 7, for example, the first andsecond sets of switches are preferably disposed on opposite sides of thetermination elements 104 and on opposite sides of the point at which thenetwork device is connected to the network bus. Thus, the first andsecond pairs of switches control the connection of the associatednetwork device(s) and the termination elements to the network bus. Inresponse to the Connect command, the bus protection element will alsopreferably close a power switch 110 disposed in-line with the power busin order to permit power to be applied to the next network device and,in particular, to the bus protection element associated with the nextnetwork device along the network bus.

[0084] While the bus protection element 94 can include a relativelysimple power switch 110 in-line with the power bus, the bus protectionelement oftentimes includes a second power detector 96 which controlsthe power switch. In these instances, the second power detector ispreferably identical to the first power detector. It should be noted,however, that the normally open switch of the second power detector isclosed, not due to the detection of power on the power bus since thesecond power detector is connected to the power bus downstream of thepower switch at a point that has not yet received power, but due to acontrol signal provided by the logic element 103, as shown in both FIGS.7 and 8. Including a second power detector is advantageous for systems10 configured as depicted in FIG. 5 in which the bus protection elementcould receive power from either direction, i.e., from either networkcontroller. Thus, the bus protection element is preferably designed tobe symmetrical so as to operate equally as well in response to power andother signals, such as commands and clock signals, being supplied fromeither direction along the network bus.

[0085] In addition to closing the power switch 110 and the second set ofswitches 108, the logic element 103 causes the termination elements 104to be switchably disconnected from the respective pairs of conductors inresponse to the Connect command so as no longer to terminate the pairsof conductors. As such, the network controller can communicate withnetwork devices located further downstream along the network bus 12. Theprocess of applying power to the power bus and individually connectingeach network device 18 to the network bus 12 via a respective busprotection element 94 is preferably continued until each of the networkdevices is connected to the network bus. The network controller and thenetwork devices then communicate as described in general above.

[0086] Before, during or following initialization, the network bus 12may suffer a bus failure, such as an open circuit, short circuit,monopolization by a network device 18 that is babbling, or the like. Aswill be understood, various types of network failures can create each ofthese conditions. The network controllers 14 a, 14 b and/or a hostcomputer 20 associated with the network controllers are adapted todetect the bus failure. For purposes of description, however, thenetwork controllers will be described as detecting the bus failure, eventhough the detection may actually be performed by the associated hostcomputer, either individually or in combination with the networkcontrollers. In this regard, the network controllers can detect a shortcircuit or an open circuit on the first or third pairs of conductors ifthe second network controller does not receive the signals transmittedby the first network controller. The network controllers can also detectan open circuit on the second pair of conductors since signalstransmitted by the network devices will be detected by only one of thenetwork controllers. Similarly, the network controllers can detect ashort circuit in the second pair of conductors since the signalstransmitted by the network devices will be received by only one, if any,of the network controllers.

[0087] The network controllers 14 a, 14 b will also be able to detect anetwork device 18 that is babbling since there will be signalstransmitted by the babbling network device on the second pair ofconductors that are not in response to a command issued by the networkcontrollers. In addition, the network controllers can detect a shortcircuit in the power bus or within a network device by detecting acurrent being carried by the power bus that exceeds a predeterminedthreshold. In addition, the network controllers can detect an opencircuit in the power bus during the process of initially applying powerto the network bus since power will not be transferred from one networkdevice on one side of the open circuit to the adjacent network device onthe other side of the open circuit and, thus, the network device withoutpower will not be able to respond to the network.

[0088] While techniques for detecting the various types of bus failuresaccording to this aspect of the present invention have been described inconjunction with a network bus 12 having a full duplex configuration,similar techniques may be employed in conjunction with a network bushaving a half duplex configuration. In addition, while techniques foridentifying the various types of bus failures have been described inconjunction with a system having a pair of network controllers, similartechniques can be employed in conjunction with other embodiments of thesystem, such as embodiments that have only a single network controller.In this regard, failures in the power bus and the second pair ofconductors are detected in the same manner as described above, whilefailures in the first and third pairs of conductors are detected by afailure of the network controller to receive the same signals that it istransmitting since the signals transmitted by the network controllerover the network bus during normal operation should be returned to thenetwork controller.

[0089] If the network controllers 14 a, 14 b detect a bus failure, thenetwork controllers remove power from the network bus 12. As such, thepower detectors 96 of each bus protection element 94 detect that powerhas been removed and allow the associated switch to return to itsnormally open position. Likewise, the power detectors signal the otherswitches of the bus protection element that power has been removed suchthat each of these other switches, i.e., the first and second sets ofswitches 98, 108 and the switches 106 associated with the terminationelements 104, also return to their normally open position.

[0090] As illustrated in the embodiment in which the system 10 includesa pair of network controllers 14 a, 14 b connected to opposite ends ofthe network bus 12, the first network controller then applies power tothe power bus and monitors the signals transmitted via the network busas a network device 18 is connected to the network bus via itsrespective bus protection element 94. Since the network bus is properlyterminated by the termination elements 104 while each respective networkdevice is initially connected to the network bus, the first networkcontroller will be able to communicate with each network device until anattempt is made to connect to another network device that is located onthe far side of an open or short circuit or until an attempt is made toconnect to a network device that is babbling or shorted. In this regard,while the network device that is positioned closest to the location ofthe bus failure and on the same side of the bus failure as the firstnetwork controller will properly connect to the network bus andcommunicate with the first network controller, any attempt to connect tothe next network device will be unsuccessful and/or will corrupt thenetwork bus.

[0091] By connecting the network devices 18 one at a time in the orderwhich the network devices are connected to the network bus 12, the firstnetwork controller 14 a can identify the location of the bus failure. Asshown in FIG. 9, for example, bus failure typically separates thenetwork bus into first and second portions 12 a, 12 b located onopposite sides of the location of the bus failure. While the first andsecond portions are no longer in direct communication with one another,the first portion of the network bus remains in communication with thefirst network controller 14 a, while the second portion of the networkbus is in communication with the second network controller 14 b. WhileFIG. 9 depicts a break in the network bus, such as resulting from anopen circuit, the failure could also be a short circuit in one or moreconductors of the network bus or a network device that has begunbabbling as described above. Each of these types of failure effectivelyseparates the network bus into first and second portions.

[0092] Once the first network controller 14 a has identified thelocation of the bus failure, the second network controller 14 b ispromoted to be a master controller so as to issue commands and totransmit a clock signal, if so desired. The second network controllerthen repeats the process previously conducted by the first networkcontroller, albeit with the network devices connected to the secondportion 12 b of the network bus 12. Thus, the second network controlleralso identifies the location of the bus failure as well as the networkdevice that is closest to the location of the bus failure on the sameside as the second network controller that can still be properlyconnected to the network bus. Although the process of inventorying thenetwork devices connected to the first and second portions of thenetwork bus is described to first be conducted by the first networkcontroller and then by the second network controller, it should beunderstood that both network controllers could, instead, proceedconcurrently.

[0093] Once the location of the bus failure has been identified by thefirst and second network controllers 14 a, 14 b, the first and secondnetwork controllers again remove power from the power bus and thensequentially connect each network device 18 to the network bus 12 viaits respective bus protection element 94. In this regard, the first andsecond network controllers connect the network devices to the networkbus in the order in which the network devices are positioned along thenetwork bus beginning with the network device that is located closest tothe respective network controller and continuing thereafter. Uponapplying power to and establishing communication with the network devicethat is located closest to the bus failure but that is still capable ofbeing properly connected to the network bus, each network controllerhalts the connection process by failing to issue another Connectcommand. Thus, the network controller does not attempt to connect to thenext network device since it was previously determined that attempts tomake a further connection will be futile in view of the bus failure.

[0094] Since the network controllers 14 a, 14 b do not issue anotherConnect command after connecting the network device 18 that is locatedclosest to the bus failure to the network bus 12 (the network devicesdesignated S4 and S5 in FIG. 9), the termination elements 104 of the busprotection element 94 associated with the network device that is locatedclosest to the bus failure will remain across the various pairs ofconductors of the network bus, thereby effectively terminating thenetwork bus. As a result of the termination of the first and secondportions of the network bus proximate the location of the bus failure,the first network controller 14 a can reliably communicate with thenetwork devices 18 connected to the first portion 12 a of the networkbus without undesirable reflections, while the second network controller14 b can reliably communicate with the network devices connected to thesecond portion 12 b of the network bus without undesirable reflections.As such, both the first and second controllers must now issue commandsover the respective portions of the network bus and must monitor theresulting responses provided by the network devices. In this regard, inorder to poll the network devices in the same sequence as set forthabove in Table 1, the first and second network controllers must now eachissue commands to the respective portion of the network bus as set forthin the following table. First Network controller Second Networkcontroller Trigger Trigger Poll S1 Poll S5 Poll S2 Trigger Trigger PollS1 Poll S6 Poll S3 Poll S7 Poll S4 Poll S8

[0095] It will be understood that the commands need not be split amongthe network controllers as illustrated in Table 2. In this regard, eachnetwork controller can issue the entire set of commands to respectiveportions of the network bus, however, network devices connected to thefirst portion of the network bus will not respond to commands issued bythe second network controller, and vice versa.

[0096] As shown in Table 2, each network controller issues a triggercommand over the respective portion of the network bus 12. In responseto the initial trigger command from the first network controller 14 a,the network devices designated S1 and S2 are polled for stored data,while in response to the initial trigger command from the second networkcontroller 14 b, the network device designated S5 is polled for storeddata. In response to the second trigger command from the first networkcontroller, the network devices designated S1, S3 and S4 are polled forstored data, while in response to the second trigger command from thesecond network controller, the network devices designated S6-S8 arepolled for stored data. Thus, the system 10 remains fully functionaleven though the network bus has failed.

[0097] While a system 10 employing a network bus 12 that extends betweena pair of network controllers 14 a, 14 b has been illustrated anddescribed, the system and method of the present invention are suitablefor a wide variety of other network bus architectures. For example, thesystem can include a network bus having a single network controllerconnected to the network bus, either at one end of the network bus or atany point along the network bus as depicted in FIG. 1. In thisembodiment, a failure along the network bus would effectively preventthe network controller from further communicating with the networkdevices 18 that are connected to the portion of the network bus on theopposite or distant side of the bus failure from the network controller.However, the network controller could still communicate with the networkdevices connected to the same portion of the network bus to which thenetwork controller is connected.

[0098] In order to continue to communicate with these network devices 18at a relative high data transfer rate, however, the network controller14 a would again inventory the network devices connected to theremaining portion of the network bus and identify the network deviceconnected to the remaining portion of the network bus and locatedclosest to the bus failure. As described above, each network device isassociated with a bus protection element 94 that has an associatedtermination element 104. As such, the network controller commands thebus protection element associated with the network device connected tothe remaining portion of the network bus and located closest to the busfailure to place the termination elements across the respective pairs ofconductors. As such, the network controller can continue to communicatewith the network devices connected to the remaining portion of thenetwork bus at a relatively high data transfer rate without sufferingfrom reflections or other noise introduced via the bus failure.

[0099] Many modifications and other embodiments of the invention willcome to mind to one skilled in the art to which this invention pertainshaving the benefit of the teachings presented in the foregoingdescriptions and the associated drawings. Therefore, it is to beunderstood that the invention is not to be limited to the specificembodiments disclosed and that modifications and other embodiments areintended to be included within the scope of the appended claims.Although specific terms are employed herein, they are used in a genericand descriptive sense only and not for purposes of limitation.

What is claimed is:
 1. A system for maintaining proper termination anderror-free communication in a network bus including a power buscomprising: at least one network device electrically connected to andadapted to communicate via said network bus, wherein said at least onenetwork device is capable of transmitting signals to said network bus,and capable of receiving signals from said network bus; and a networkcontroller electrically connected to said network bus, wherein saidnetwork controller is adapted to direct communications with said atleast one network device via said network bus, and wherein said networkcontroller is adapted to provide power signals to said at least onenetwork device via the power bus of the network bus; and at least onebus protection element disposed between said at least one network deviceand the network bus, wherein at least one of said at least one busprotection element and said network controller is capable of monitoringsignals on the network bus to thereby identify improper signals on thenetwork bus, and wherein the at least one bus protection element iscapable of selectively connecting and disconnecting the at least onenetwork device to and from the network bus based upon said at least onebus protection element and said network controller identifying apredefined number of improper signals on the network bus.
 2. A systemaccording to claim 1, wherein said at least one bus protection elementis capable of monitoring the signals transmitted by said at least onenetwork device and the signals received by said at least one networkdevice, wherein said at least one bus protection element is capable ofidentifying improper signals transmitted by said at least one networkdevice and controllably halting the transmission of signals fromrespective network devices onto the network bus if a predefined numberof improper signals are identified.
 3. A system according to claim 2,wherein each bus protection element comprises: at least one isolationswitch disposed between said at least one network device and the networkbus, wherein said at least one isolation switch is capable ofcontrollably halting the transmission of signals from said at least onenetwork device onto the network bus; and at least one logic elementelectrically connected to said at least one isolation switch and said atleast one network device and capable of monitoring the signalstransmitted by said at least one network device and the signals receivedby said at least one network device, wherein said at least one logicelement is capable of identifying improper signals transmitted by saidat least one network device and controllably operating said at least oneisolation switch if a predefined number of improper transmitted signalsare identified.
 4. A system according to claim 2, wherein each networkdevice includes a communications interface comprising: a transmitter fortransmitting signals from the network device to the network bus; and areceiver for receiving signals on behalf of the network device from thenetwork bus, wherein said at least one bus protection element is capableof monitoring the signals transmitted by the transmitter and the signalsreceived by the receiver.
 5. A system according to claim 4 furthercomprising: at least one diagnostic switch electrically connected to anddisposed between the at least one transmitter and said at least onenetwork device, wherein said at least one diagnostic switch iselectrically connected to said at least one bus protection element, andwherein said at least one diagnostic switch is capable of controllablyhalting the transmission of signals from a respective network device tothe respective transmitter, wherein each bus protection element iscapable of controllably operating a respective diagnostic switch if apredefined number of improper transmitted signals are identified, thetransmitter and receiver associated with a respective network device areelectrically connected such that after the bus protection elementcontrollably operates the respective diagnostic switch to halt thetransmission of signals from the respective network device the busprotection element is capable of transmitting diagnostic signals andreceiving corresponding return signals, and wherein the bus protectionelement is capable of comparing the diagnostic signals and the returnsignals such that when the diagnostic signals and the return signals aresubstantially identical the bus protection element is capable ofcontrollably operating the respective at least one isolation switch andthe respective diagnostic switch to permit the transmission of signalsfrom the respective network device.
 6. A system according to claim 1,wherein each bus protection element comprises: a first set of switchesin-line with the network bus that are closed upon application of powerto the power bus; a second set of switches in-line with the network busthat are closed following the application of power to the power bus andin response to a command from said network controller; and a terminationelement adapted to be switchably connected to the network bus upon theapplication of power to the power bus and to be switchably disconnectedfrom the network bus in response to a command from said networkcontroller.
 7. A system according to claim 6, wherein bus protectionelement further comprises a first power detector for detecting theapplication of power to the power bus and, if power is applied to thepower bus, for signaling said first set of switches to close and saidtermination element to be connected to the network bus.
 8. A systemaccording to claim 6, wherein each bus protection element transmits aresponse to said network controller once said first set of switches havebeen closed and said termination element has been switchably connectedto the network bus.
 9. A system according to claim 6 further comprisinga second network controller, wherein said network controllers areconnected to opposite ends of the network bus.
 10. A system according toclaim 6 wherein each network device is also associated with a networkdevice interface connected to the network bus and connected to thenetwork device via a data channel, said network device interfaceincluding said selective connection device.
 11. A method for maintainingproper termination and error-free communication conducted via a networkbus that is electrically connected to at least one network device,wherein the network bus includes a power bus, said method comprising:monitoring signals on the network bus; identifying a predefined numberof improper signals from the signals on the network bus; and switchablydisconnecting the at least one network device after identifying apredefined number of improper signals.
 12. A method according to claim11, wherein monitoring comprises monitoring signals transmitted by theat least one network device to the network bus and the signals receivedby the at least one network device from the network bus, whereinidentifying comprises identifying a predefined number of impropersignals from the signals transmitted by the at least one network deviceto the network bus, and wherein switchably disconnecting compriseshalting the transmission of signals from the at least one network deviceafter identifying the predefined number of improper signals.
 13. Amethod according to claim 12, wherein each network device includes atransmitter for transmitting signals from the network device to thenetwork bus, and a receiver for receiving signals on behalf of thenetwork device from the network bus, wherein each respective transmitterand receiver are electrically connected, said method further comprising:transmitting diagnostic signals through the respective transmitter andthereafter receiving corresponding return signals from the respectivereceiver after halting the transmission of signals from the respectivenetwork device; comparing the diagnostic signals and the return signals;and permitting the transmission of signals from the respective networkdevice to the network bus, wherein said permitting the transmission ofsignals comprises permitting the transmission of signals when thediagnostic signals and the return signals are substantially identical.14. A method according to claim 11 further comprising: detectingapplication of power signals to the power bus; closing a first set ofswitches that are in-line with the network bus upon detecting theapplication of power to the power bus to thereby enable a first networkdevice to communicate with a network controller via the network bus;switchably connecting a termination element to the network bus upondetecting the application of power to the power bus to maintain theproper termination of the network bus; closing a second set of switchesthat are also in-line with the network bus upon receipt of a commandfrom a network controller via the network bus; and switchablydisconnecting the termination element from the network bus upon receiptof a command from the network controller via the network bus to enablethe network controller to communicate with another network device,wherein detecting, closing, switchably connecting, closing andswitchably disconnecting all occur before monitoring signals on thenetwork bus.
 15. A method according to claim 14 further comprisingclosing a power switch that is in-line with the power bus upon receiptof the command from the network controller.
 16. A method according toclaim 14, wherein closing the second set of switches and switchablydisconnecting the termination element from the network bus are inresponse to the same command from the network controller.
 17. A methodaccording to claim 14 further comprising transmitting a response to thenetwork controller once the first set of switches is closed and thetermination element is switchably connected to the network bus.
 18. Amethod according to claim 14, wherein switchably connecting thetermination element comprises switchably connecting the terminationelement to the network bus between the first and second sets ofswitches.
 19. A method according to claim 14, wherein the network buscomprises the power bus and at least one pair of conductors, whereinswitchably connecting the termination element comprises switchablyconnecting a respective termination element across each pair ofconductors.
 20. A bus protection element for operation with a networkdevice electrically connected to and adapted to communicate via a bus,wherein the network device includes a communications interface having atransmitter for transmitting signals from the network device to the bus,and a receiver for receiving signals on behalf of the network devicefrom the bus, said bus protection element comprising: at least oneisolation switch disposed between the communications interface and thebus, wherein said at least one isolation switch is capable ofcontrollably halting the transmission of signals from the network deviceonto the bus; and a logic element electrically connected to said atleast one isolation switch and the communications interface and capableof monitoring the signals transmitted by the transmitter and the signalsreceived by the receiver, wherein said logic element is capable ofidentifying improper signals transmitted by the transmitter andcontrollably operating said at least one isolation switch therebyhalting the transmission of signals from the network device if apredefined number of improper transmitted signals are identified.
 21. Abus protection element according to claim 20, wherein the predefinednumber of improper signals is dependant upon a predetermined number ofsignals transmitted by the transmitter, and wherein said logic elementis capable of identifying the predetermined number of signalstransmitted by the transmitter and controllably operating said at leastone isolation switch thereby halting the transmission of signals fromthe network device if a predefined number of improper transmittedsignals are identified.
 22. A bus protection element according to claim20 further comprising: two diagnostic switch, wherein one diagnosticswitch is disposed between the transmitter and the network device andthe other diagnostic switch is disposed between the receiver and thenetwork device, and wherein said diagnostic switches are capable ofcontrollably halting the transmission of signals from the network deviceto the transmitter and the reception of signals by the network devicefrom the receiver, wherein said logic element is capable of controllablyoperating said diagnostic switches if a predefined number of impropertransmitted signals are identified, wherein the transmitter and receiverare electrically connected such that after said logic elementcontrollably operates said diagnostic switches to halt the transmissionof signals from the network device and the reception of signals by thenetwork device said logic element is capable of transmitting diagnosticsignals through the transmitter and receiving corresponding returnsignals from the receiver, and wherein said logic element is capable ofcomparing the diagnostic signals and the return signals such that whenthe diagnostic signals and the return signals are substantiallyidentical said logic element is capable of controllably operating saidat least one isolation switch and said diagnostic switch to permit thetransmission of signals from the network device to the bus via thetransmitter.
 23. A bus protection element according to claim 20, whereinsaid at least one isolation switch operates in at least one of an onmode wherein said at least one isolation switch permits the networkdevice to transmit signals to the bus and an off mode wherein said atleast one isolation switch prevents the network device from transmittingsignals to the bus, and wherein said logic element is capable ofcontrollably operating the mode of said at least one isolation switch.24. A bus protection element according to claim 23, wherein said logicelement is capable of operating said at least one isolation switch inthe on mode before identifying the predefined number of impropersignals, and wherein said logic element is capable of placing said atleast one switch in the off mode after identifying the predefined numberof improper signals.
 25. A bus protection element according to claim 24further comprising: a diagnostic switch disposed between the transmitterand the network device, wherein said diagnostic switch operates in atleast one of an on mode wherein said diagnostic switch permits thenetwork device to transmit signals to the transmitter and an off modewherein said at least one isolation switch prevents the network devicefrom transmitting signals to the transmitter, wherein said logic elementis capable of controllably operating said diagnostic switch in the onmode before identifying the predefined number of improper signals,wherein said logic element is capable of placing said diagnostic switchin the off mode after identifying the predefined number of impropertransmitted signals, wherein the transmitter and receiver areelectrically connected such that after said logic element controllablyplaces said diagnostic switch in the off mode said logic element iscapable of transmitting diagnostic signals through the transmitter andreceiving corresponding return signals from the receiver, and whereinsaid logic element is capable of comparing the diagnostic signals andthe return signals such that when the diagnostic signals and the returnsignals are substantially identical said logic element is capable ofplacing said at least one isolation switch and said diagnostic switch inthe on mode.
 26. A bus protection element for maintaining propertermination and error-free communication in a network bus including apower bus, said bus protection element associated with at least onenetwork device and adapted to selectively connect each associatednetwork device to the network bus, said bus protection element also incommunication with a network controller via the network bus, said busprotection element comprising: a first set of switches in-line with thenetwork bus that are closed upon application of power to the power bus;a second set of switches in-line with the network bus that are closedfollowing the application of power to the power bus and in response to acommand from the network controller; and a termination element adaptedto be switchably connected to the network bus upon the application ofpower to the power bus and to be switchably disconnected from thenetwork bus in response to a command from the network controller.
 27. Abus protection element according to claim 26 further comprising a firstpower detector for detecting the application of power to the power busand, if power is applied to the power bus, for signaling said first setof switches to close and said termination element to be connected to thenetwork bus.
 28. A bus protection element according to claim 27 furthercomprising a power switch in-line with the power bus that is closed inresponse to the command from the network controller.
 29. A busprotection element according to claim 28 further comprising a secondpower detector including said power switch for detecting the applicationof power to the power bus and, if power is applied to the power bus, forsignaling said second set of switches to close.
 30. A bus protectionelement according to claim 26, wherein said second set of switches areclosed and said termination element is disconnected from the network busin response to the same command from the network controller.
 31. A busprotection element according to claim 26, wherein said terminationelement is switchably connected to the network bus between said firstand second sets of switches.
 32. A bus protection element according toclaim 26, wherein the network bus comprises the power bus and at leastone pair of conductors, wherein the bus protection element comprises thesame number of termination elements as the number of pairs ofconductors, and wherein each termination element is adapted to beswitchably connected across a respective pair of conductors.
 33. A busprotection element according to claim 26, wherein said terminationelement comprises a resistor having a resistance that matches acharacteristic impedance of the network bus.